ap valve exterior to the pump. If delivery valve
seats be put in the mouth of the air pump at all, the best mode of fixing
them appears to be that adopted by Messrs. Maudslay. The top of the pump
barrel is made quite fair across, and upon this flat surface a plate
containing the delivery valve is set, there being a small ledge all round
to keep it steady. Between the bottom of the stuffing box of the pump cover
and the eye of the valve seat a short pipe extends encircling the pump rod,
its lower end checked into the eye of the valve seat, and its upper end
widening out to form the bottom of the stuffing box of the pump cover. Upon
the top of this pipe some screws press, which are accessible from the top
of the stuffing box gland, and the packing also aids in keeping down the
pipe, the function of which is to retain the valve seat in its place. When
the pump bucket has to be examined the valve seat may be slung with the
cover, so as to come up with the same purchase. For the bucket valves of
such pumps Messrs. Maudslay employ two or more concentric ring valves with
a small lift. These valves have given a good deal of trouble in some cases,
in consequence of the frequent fracture of the bolts which guide and
confine the rings; but this is only a fault of detail which is easily
remedied, and the principle appears to be superior to that of any of the
other metallic air pump valves at present in common use.

[Illustration: Fig. 41. TRUNK AIR PUMP. Scale 3/4 inch to 1 foot.]

457. _Q._--Are not air pump valves now very generally made of india rubber?

_A._--They are almost invariably so made if the engines are travelling
fast, as in the case of direct acting screw engines, and they are very
often made of large discs or rings of india rubber, even when the engines
travel slowly. A very usual and eligible arrangement for many purposes is
that shown in fig. 41, where both foot and delivery valves are situated in
the ends of the pump, and they, as well as the valve in the bucket are made
of india rubber rings closing on a grating. The trunk in the air pump
enables guide rods to be dispensed with.

[Illustration: Fig. 42. PENN'S DISK VALVE FOR AIR PUMP. Section.]

[Illustration: Fig. 43. PENN'S DISK VALVE FOR AIR PUMP. Ground Plan.]

[Illustration: Fig. 44. MAUDSLAY'S DISC VALVE FOR AIR PUMP. Section.]

458. _Q._--The air pump, when double acting, has of course inlet and outlet
valves at each end?

_A._--Yes; and the general